Assembly for measuring a magnetic field, using a bridge circuit of spin tunnel elements and a production method for the same

ABSTRACT

The invention relates to an assembly for measuring a magnetic field, comprising at least a first layer assembly ( 8 ) and at least a second layer assembly ( 10 ), whereby the first layer assembly ( 8 ) and the second layer assembly ( 10 ) have a hard magnetic layer ( 16 ), a layer ( 18 ) which acts as a tunnel barrier and lies adjacent to the hard magnetic layer and a soft magnetic layer ( 20 ) which lies adjacent to the layer ( 18 ) which acts as a tunnel barrier. The layer assemblies ( 8, 10 ) are located in a bridge circuit for determining the electric resistance. The invention is characterized in that the second layer assembly ( 10 ) also has an electrically conductive anti-ferromagnetic layer ( 22 ) lying adjacent to the soft magnetic layer ( 20 ), or an electrically conductive artificial anti-ferromagnet lying adjacent to said soft magnetic layer ( 20 ). The invention also relates to a method for producing the inventive assembly.

The invention relates to an arrangement for measuring a magnetic fieldwith at least one first layer assembly and at least a second layerassembly whereby the first layer assembly and the second layer assemblyhave a hard magnetic layer, a layer which is effective as a tunnelbarrier adjacent the hard magnetic layer and a soft magnetic layeradjacent the layer which is effective as the tunnel barrier, and wherebythe layer assemblies are arranged in a bridge circuit for determiningthe electrical resistance. The invention further relates to a method ofmaking an arrangement for measuring a magnetic field in which the atleast one first layer assembly and at least one second layer assemblyare produced, whereby the first layer assembly and the second layerassembly have a hard magnetic layer, a layer effective as a tunnelbarrier adjacent the hard magnetic layer and a soft magnetic layeradjacent the layer effective as the tunnel barrier.

An arrangement of the kind described and a process of the kind describedare known. In the publication INFO PHYS TECH No. 24, October 1999, BMBF,VDI Technology Center Physical Technologies there is described how sucharrangements can be used for measuring magnetic fields. A preferredfield of use is for example the detection and the measurement ofmovements which are generally of great significance in automobiletechnology and in automation technology. Conventionally for suchposition sensing, Hall sensors are used (generally ABS sensors inautomobiles). A new sensor concept utilizes the so-called travellingmagneto resistance effect (GMR-Effect “Giant Magnetoresistance”). Thiseffect is based upon the phenomenon that electrons because of theirquantized spin states are strongly dispersed differently upon passagethrough magnetic layers each in dependence upon the magnetic orientationof these layers. With parallel magnetization, the dispersion is lessthan with magnetization which is arranged antiparallel. This gives riseto a change in the electrical resistance as a function of the externalmagnetic field in which the layer arrangement finds itself. Thesignificance for a position sensor is that the outer magnetic field canbe influenced by a movable element (generally a rotor or also alinearly-movable element) whereby these influences can be indicated asthe electrical resistance in the layer arrangement.

Basically the layer construction is so chosen that one magnetic layer isconfigured as a measuring layer and another magnetic layer as areference layer. With this arrangement, which is based upon the GMReffect, the intervening layer between these magnetic layers is a metallayer. If one replaces the metallic intermediate layer in the layerstructure by a thin electrically-insulating layer, like for exampleAl₂O₃, one can obtain a magnetic tunnel component in which the tunnelcurrent is switched in a manner similar to the current in the metallicGMR element. The advantage of the TMR element resides in a still highersignal level and in an extremely low requirement for an active componentarea.

There are high requirements for the measurement of the resistance of thelayer arrangement since the resistance is the decisive parameter for theconclusion as to the magnitude and/or the direction of the magneticfield to be measured. It is known that resistances can be measuredespecially precisely with a Wheatstone bridge circuit. This is mainlybased in temperature compensation requirements. Additionally with aWheatstone bridge there is an output voltage which is symmetrical withrespect to a zero point. In the construction of a Wheatstone bridge, thequality of the measurement is dependent upon the precision of theresistances used. Fixed resistances are determinable with practicallyoptional precision. If one however utilizes a plurality of layerarrangements in the bridge circuit, their electrical resistances must bedetermined with precision or established with precision.

The invention has as its object, therefore, to provide an arrangementfor measuring a magnetic field and a manufacturing process such that theelectrical characteristics of the elements participating in themeasurement can be known precisely or established with precision. Theseobjects are achieved with the features of claims 1 and 12.

The invention builds upon the arrangement described at the outset inthat the second layer assembly has, in addition, anelectrically-conducting antiferromagnetic layer adjacent the softmagnetic layer or an electrically-conductive synthetic antiferromagnetadjacent the soft magnetic layer. In this manner, the second layerassembly, which is used as a resistor in the Wheatstone bridge circuit,can be designed like the first layer assembly. By means of theadditional electrically-conductive antiferromagnetic layer, the secondlayer assembly is rendered magnetically insensitive. Such a “pinning”with the second layer assembly has significant advantages by comparisonto a magnetic shielding. The last, which also can be used to reduce themagnitude sensitivity, requires the deposition of thicker high μ layersin the micrometer range. This has significant disadvantages in theproduction of the components. Preferably the bridge circuit is aWheatstone bridge circuit.

In an embodiment of the invention, the four resistances of theWheatstone bridge circuit are configured as first or second layerassemblies with at least one of the layer assemblies configured with thesecond layer assembly. With such a “full bridge”, a greater stroke ofthe measurement signal can be generated.

In another embodiment, one resistance of the Wheatstone bridge isconfigured as a first layer assembly, one resistance of the Wheatstonebridge is configured as a second layer assembly and two resistances ofthe Wheatstone bridge are configured as fixed resistors. Since the twofixed resistors do not contribute to the bridge output signal of the“half bridge” the attainable stroke of the bridge is reduced, althoughwith a precision selection of the fixed resistance, there can be abetter reduction of the offset (signal voltage without applied fieldconditioned on resistance fluctuations over the wafer) of the bridge.While this can be realized in conjunction with tunnel contacts notwithout problems, it is conceivable for the hard magnetic layer of thelayer arrangement to be an electrically-nonconductive antiferromagneticlayer. In this manner a reference magnetization can be supplied which isonly limitedly influenced by the controlling outer magnetic field.

It can be advantageous to pin the hard magnetic layer of the layerassembly with an electrically-nonconductive antiferromagnetic layer. Thehard magnetic layer can be selected in such manner that the bestcharacteristics with respect to the tunnel contact are obtainable whileit contains because of the pinning with the antiferromagnet, asufficient counterfield stability and thus can be affected only in amagnetic manner.

It can be advantageous for the electrically-nonconductiveantiferromagnetic layer to be comprised of NiO_(x). NiO_(x) is suitableas a nonconductor since in tunnel contacts, the current flow runsperpendicular to the substrate.

In another embodiment, the electrically-nonconductive antiferromagneticlayer is comprised of a synthetic antiferromagnet. Syntheticantiferromagnets (AAF) have good counterfield stability especially athigh operating temperatures.

It is preferable for the synthetic antiferromagnet to have a layersystem of Cu and Co or Ru. By the appropriate choice of the Cu layerthickness the antiparallel locations of the Co layers and Ru layers areachieved which can only be removed at very high magnetic fields. If oneselects the thickness of the Co or Ru layers additionally such that theresulting magnetization is small, then the preferred magnetizationdirection of the synthetic antiferromagnet can only be changed withdifficulty in an external magnetic field.

Preferably the electrically conductive antiferromagnetic layer iscomprised of IrMn. In this manner it can act simultaneously to providethe requisite electrical conductivity function and also has theantiferromagnetic serving for pinning action. Preferably the tunnelbarrier is comprised of Al₂O₃. Advantageously, the layer is so made thata thin Al layer is sputtered onto the hard magnetic layer. The sputteredAl layer is then reacted with an oxygen atmosphere of about 10 m bar forabout 30 minutes, whereby a mercury low-pressure lamp is used togenerate UV radiation and has the effect of converting the molecularoxygen into high reactive atomic oxygen and ozone. The oxidation is socarried out that as much as possible all of the deposited Al iscompletely oxidized. A plasma oxidation of the Al is also possible.

The invention includes a process in which the layer arrangements areformed parallel to one another in a single procedure and on the secondlayer arrangement additionally, an electrically-conductive antimagneticlayer is provided adjacent the soft magnetic layer or anelectrically-conductive synthetic antiferromagnet is provided on andadjacent the soft magnetic layer. Through this process it is achievedthat both the first and the second layer arrangements have the sameresistance and electrical characteristics. This has been found to beespecially suitable for use in a Wheatstone bridge circuit.

Preferably the layer arrangements are produced by deposition of thelayers. This is an approved method of producing such components.

It is especially preferably when the additional electrically-conductiveantiferromagnetic layer is applied in conjunction with the tunnelcontact layer system. The fabrication process of the layer arrangementcan thus be effected without interruption.

Then the finished layer assemblies, preferably are connected in a bridgecircuit.

The invention is based upon the surprising discovery that themeasurement of magnetic fields through the additional application ofelectrically-conductive antiferromagnetic layer on the magnetic layer inconjunction with a bridge circuit gives especially precise results. Thebasis for this, among other things, is that identical layer assemblieswith identical electrical characteristics can be used both as referenceelements and as magnetic field sensitivity elements. This has a specialadvantage also from the point of view of the process since theadditional conductive antiferromagnetic layer of different layerassemblies can be produced in the same fabrication process.

The invention is described with reference to the accompanying drawingfor exemplary embodiments.

FIG. 1 shows a first layer assembly for an arrangement in accordancewith the invention;

FIG. 2 shows a second layer assembly for an arrangement according to theinvention;

FIG. 3 shows the arrangements according to the invention.

In FIG. 1 a first layer assembly 8 is shown for an arrangement inaccordance with the invention. On a substrate 12, a nonconducting orconducting antiferromagnetic layer 14 is disposed, for example, asynthetic antiferromagnet. This layer 14 can be formed for example byNiO_(x) or IrMn. On this layer is found a hard magnetic layer 16. Thisis followed by a tunnel barrier layer 18, for example of Al₂O₃. On thelayer 18, effective as a tunnel barrier, there is finally a softmagnetic layer 20.

In FIG. 2, a second layer assembly 10 is provided for use in thearrangement of the invention and the layer sequence is identical exceptthat in the second layer arrangement 10, there is additionally on thesoft magnetic layer 20 an electrically-conductive antiferromagneticlayer 22, for example of IrMn. Even including the soft magnetic layer20, the layer assemblies 8 and 10 of FIGS. 1 and 2 can be fabricated ina common process. This has fabrication advantages; further, the layerassemblies 8, 10 are identical up to the layer 20, especially in termsof their electrical characteristics.

In FIG. 3 the Wheatstone bridge 24 is shown for measuring the magneticfield dependency resistance. In the present case, it is a “half bridge”,that means that the resistances R3 and R4 are fixed resistances whilethe resistances R1 and R2 are layer assemblies. If one selects, forexample, for the resistance R1 the layer arrangement 8 according to FIG.1 and the layer arrangement 10 for the resistance R2 according to FIG.2, which have been produced in the same fabrication process, preciseresistance measurements of those precise magnetic field measurements canbe effected.

In the present description, in the drawing as well as in the claims, thefeatures of the invention can be taken individually and also in optionalcombination and are important to the inventor's concept.

What is claimed is:
 1. An arrangement for measuring a magnetic fieldwith at least one first layer assembly (8) and at least one second layerassembly (10), whereby the first layer assembly (8) and the second layerassembly (10) have a hard magnetic layer (16), a layer (18) effective asa tunnel barrier and adjacent the hard magnetic layer, and a layer (18)effective as a tunnel barrier layer adjacent the soft magnetic layer(20) and whereby the layer assemblies (8, 10) are arranged in a bridgecircuit (24) for determining the electrical resistance, characterized inthat the second layer assembly (10) additionally has an electricallyconductive antiferromagnetic layer (22), adjacent the soft magneticlayer (20) or an electrically-conductive synthetic antiferromagnet isadjacent the soft magnetic layer (20).
 2. The arrangement according toclaim 1, characterized in, that the bridge circuit (24) is a Wheatstonebridge circuit.
 3. The arrangement according to claim 2, characterizedin, that each of the four resistances of the Wheatstone bridge circuit(24) is configured with a first layer assembly (8) or second layerassembly (10) whereby at least one of the layer assemblies is configuredas the second layer assembly (10).
 4. The arrangement according to claim2, characterized in, that one resistance (R₁) of the Wheatstone bridgeis configured as the first layer assembly (8), a resistance (R₂) of theWheatstone bridge circuit (24) is configured as the second layerassembly (10) and that two resistances (R₃ and R₄) of the Wheatstonebridge circuit 24 are configured as fixed resistors.
 5. The arrangementaccording to claim 1, characterized in, that the hard magnetic layer(16) of the layer assemblies (8, 10) is an electrically nonconductiveantiferromagnetic layer (14).
 6. The arrangement according to 1,characterized in, that the hard magnetic layer (16) of the layerassemblies (8, 10) is penned with an electrically nonconductiveantiferromagnetic layer (14).
 7. The arrangement according to claim 6,characterized in, that the electrically nonconducting antiferromagneticlayer (14) is comprised of NiO_(x).
 8. The arrangement according toclaim 6, characterized in, that the electrically nonconductingantiferromagnetic layer (14) is comprised of a syntheticantiferromagnet.
 9. The arrangement according to claim 8, characterizedin, that synthetic antiferromagnet has a layer system of Cu and Co. 10.The arrangement according to claim 1, characterized in, that theelectrically conducting antiferromagnetic layer (22) is comprised ofIrMn.
 11. The arrangement according to claim 1, characterized in, thatthe layer effective as a tunnel barrier (18) is comprised of Al₂O₃. 12.A method of producing an arrangement for measuring a magnetic field inwhich at least one first layer assembly (8) at least one second layerassembly (10) are produced, whereby the first layer assembly and thesecond layer assembly have a first hard magnetic layer (16), a layer(18) effective as a tunnel barrier and adjacent the hard magnetic layer(16) and a soft magnetic layer (20) adjacent the layer (18) effective asthe tunnel barrier, characterized in that the layer assemblies (8, 10)are produced in parallel in a single process and that on the secondlayer assembly (10) an additional electrically conductingantiferromagnetic layer is provided on the soft magnetic layer (20) oran electrically-conducting antiferromagnet is provided on the softmagnetic layer (20).
 13. The method according to claim 12, characterizedin that the layer assemblies (8, 10) are produced by deposition oflayers.
 14. The method according to claim 13, characterized in that theadditional electrically-conducting antiferromagnetic layer (22) isapplied in conjunction with the deposition.
 15. The method according toclaim 13, characterized in that the applied layer assemblies (8, 10) areconnected in a bridge circuit (24).
 16. The method according to claim12, characterized in that the layer assemblies (8, 10) before finishingare connected in the bridge circuit (24) and that on them as necessarythe electrically-conducting antiferromagnetic layer (22) is applied tothe desired elements of the bridge circuit (24).